One of the limiting factors in the design of high performance gas turbine engines is the ability of the turbine wheel or nozzle assembly to withstand the severe conditions of temperature and stress which exist during operation of the engine.
Turbine wheels and nozzles are located immediately downstream from the combustion area of an engine and must operate in an environment of high temperature corrosive gases. In addition, the turbine wheels operate under great mechanical stress due to their very high rotational speeds--often exceeding 100,000 revolutions per minute.
In the past, it has been generally known to cast turbine wheel assemblies in one piece. See, for example, U.S. Pat. Nos. 3,283,377 and 3,312,449. However, even though special care was taken to produce high quality, fine grained hub castings, defects initiated in the airfoil portion often led to failures.
One way to reduce failures is to forge the wheel hub or disk from a high strength alloy and then mechanically attach individual blades to it. The individual blades can be cast with high precision and inspected for quality before assembly thus reducing the probability of a defect in the wheel. In addition, such small castings may be directionally solidified in a columnar grain structure or even solidified as a single crystal to further improve their high temperature properties. See, for example, U.S. Pat. Nos. 3,342,455: 3,680,625; 3,714,977; 3,260,505 and 3,376,915.
However, this two-stage process is very expensive and time consuming; so attempts have been made to achieve the improved blade structure in an integral casting. U.S. Pat. No. 3,614,976 suggests that rotation of a casting mold during solidification can result in columnar grained blades and equiaxed fine grains in the hub of turbine wheels. A different approach is suggested by U.S. Pat. No. 3,741,821 wherein a forged wheel assembly (having an all equiaxed grain structure) is heat treated only in the blade region to allow those grains to grow into a larger and/or columnar structure.
More recently, attempts have been made to more accurately control the solidification of a cast wheel assembly to produce the desired dual macrostructures. See, for example, U.S. Pat. Nos. 3,283,377; 3,312,449; 3,598,169; 4,240,495 and 4,436,485.
A major disadvantage of these prior art processes is that it is still very difficult to precisely control the thermal gradients in the mold, and thus the solidification process, to achieve the desired microstructure in the as-cast turbine wheel.
It is, therefore, an object of the present invention to provide an improved method and apparatus for controlling the solidification of a cast disk-shaped component.